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1 8月 2018
Making our cities more resilient to earthquakes is as much about how you manage them as it is how you build them.
San Francisco was largely destroyed at 5:12 am on 18 April 1906. Rupturing 296 miles of the San Andreas fault, the 8.3 magnitude earthquake left a trail of devastation in its wake. Official statistics estimate that up to 3,000 people were killed by the quake, 225,000 people were made homeless, and 28,000 buildings were ruined. Photos show the City Hall in pieces, tram tracks upturned and fire ravaging its way through the remaining structures. The US National Oceanic and Atmospheric Administration says the Californian quake and subsequent fires caused $400m of damage – around $10bn in 2017 valuations.
While the earthquake of 1906 has long been surpassed in power – a 1960 quake in Chile measured 9.5 – it taught unprepared city planners and seismologists about the catastrophic power the Earth possesses. It is predicted that another 1906-scale earthquake will hit San Francisco within the next 30 years but, despite advances in technology, it is not possible to forecast when.
Recent earthquake research has focused on detecting when one has occurred to prepare people for its impact. Two types of seismic waves are created: primary (P-waves) and secondary (S-waves). P-waves travel at about 7km per second and make the rocks around them vibrate with the direction of movement. S-waves move at slower speeds but in a shearing motion that makes rocks vibrate at a right angle, causing surface damage.
Precious seconds allow time to ensure car drivers do not enter tunnels or start crossing bridges, and a train driver to apply the brakes.
Earthquake early warning systems (EWS) are able to operate using the differences in wave type. “We try to detect the earthquake on the P-wave and send out a warning before the more damaging S-waves arrive,” explains Angela Chung, a postdoctoral researcher at the University of California’s Berkeley Seismological Laboratory. Chung is working on the algorithm at the heart of the state’s new EWS, ShakeAlert. Within the system, sensors in at least four monitoring stations have to detect a P-wave before a notification is sent out, Chung says. The P-wave information allows the system to automatically estimate the location and magnitude of the earthquake. It is then able to project the amount of ground shaking there will be. “Knowing that shaking is about to occur means people can take precautionary action in those few seconds,” she adds.
It is just enough time for people to drop to the floor, find cover, hold on to a stable item or move away from glass windows. These precious seconds allow time to ensure car drivers do not enter tunnels or start crossing bridges, and a train driver to apply the brakes.
The plan is for ShakeAlert to eventually be able to send notifications of upcoming seismic activity to people’s mobile phones. Current technological limitations in sending these to millions of devices at once – such as patchy network coverage – mean the system is likely to use TV and radio to spread the message initially. California’s EWS remains in a beta stage of testing at the moment, but two other countries, Mexico and Japan, already have working systems.
Mexico City’s SASMEX alert system is one of the world’s oldest EWSs; it started working in 1991, six years after a devastating earthquake hit the region. There is a smartphone app but most alerts are issued through speakers in public buildings and parks. The alarm system does not say how strong an earthquake is but is able to detect seismic activity happening 200 miles away on the Pacific coast. The distance the disruptive S-waves have to travel meant that during the September 2017 7.1 magnitude earthquake that hit the country, those in the capital had 60 to 90 seconds to prepare for impact.
Japan’s nationwide alert system, J-Alert, is perhaps the world’s most advanced and extensive EWS. It uses loudspeakers, TV and radio broadcasts, mobile phone networks and email to disseminate warnings across the country. It has also helped to provide tsunami warnings and has recently been used to inform citizens of North Korean ballistic missile test flights passing overhead.
“With the increasing impact of global warming on extreme natural hazards, EWSs are increasingly required to cater for multiple hazards or even cascading hazards [or chain reactions],” explains Carina Fearnley, a lecturer in science and technology studies at University College London. “Even 30-50 seconds’ warning can make a huge difference, but ultimately, when it comes to earthquakes, it is about having good building codes, having plans in place and being prepared, and using any EWS to help reduce the death toll and damage as much as possible.”
Despite Mexico’s EWS, more than 300 people died during the colossal September 2017 earthquake. The country has a strong series of seismic building codes in place, which implement laws designed to toughen up constructions to potential earthquakes. Dramatic videos from the scene of September’s earthquake show buildings collapsing as the earth beneath them shook.
“The biggest challenge in earthquake engineering is the vulnerability of old building stock that does not meet current codes and standards,” says Ziggy Lubkowski, EMEA seismic business and skills leader at Arup. Like Mexico, many of the world’s countries have their own construction standards and building codes when it comes to seismic resilience.
Seismic codes can vary in their requirements, but in most cases they require a building to be able to withstand the theoretical strongest earthquake in its geography. Buildings are not expected to be damage free – this is virtually impossible – but should remain standing.
Flavia De Luca, a lecturer from the Department of Civil Engineering at the University of Bristol, argues it is possible to make an earthquake-resistant building out of any material – reinforced concrete, timber, steel, masonry and more – if the proper procedures are followed. “The critical aspect is that in many cases there are codes, there are proper rules to build up structures that are earthquake resistant, but the execution and the building practice is poor or not properly controlled. This problem affects many developing and developed countries,” she explains. Following the central Mexico earthquake, residents complained that developers may not have followed the regulations when building their apartments – more than 3,000 structures collapsed.
As well as the potential risk of not being followed by construction companies and architects, earthquake building regulations can take a long time to filter through to the structures in a city. “One of the most difficult problems is that even if we revise our building code, it may only be effective tens of years later, because most existing buildings are designed according to the previous code,” argues Koichi Kusunoki, associate professor at the University of Tokyo’s Earthquake Research Institute.
Kusunoki adds that the screening of a building’s seismic capacity allows it to be analysed against codes and then retrofitted if it is needed. The 53-storey Shinjuku Nomura Building in central Tokyo, which was completed in 1978, has been one recipient of Japan’s retrofitting. In 2015, the building had two mass dampers installed on its 52nd and 53rd floors. These dampers are essentially giant 700 tonne weights, designed to move in the opposite direction to vibrations from earthquakes. The effect of this is to cancel out the building’s movement, with the dampers absorbing an earthquake’s seismic energy.
The Japanese government claims that if the 9.1 magnitude earthquake of 2011 was repeated, it is expected that the Shinjuku Nomura Building would now soak up 20%-25% of the vibrations, and the duration of the period in which the building moves would be reduced by 50%.
The impact that an earthquake has on developing nations can be particularly catastrophic. The 7.3 magnitude earthquake that shook the Iran-Iraq border in November 2017 claimed more than 500 lives. Iran’s president, Hassan Rouhani, blamed the high death toll on the poor standard of some of the government-built buildings that collapsed.
The Nepalese capital, Kathmandu, was largely destroyed by a 7.5 magnitude quake in April 2015. “In Nepal, the preparedness and resilience was relatively low when the earthquake hit,” explains Rabindra Singh, project manager at international development charity Practical Action. “There were no systematic plans and strategic interventions from government. Whatever little policies it had were not fully enforced.”
The governments of developing nations need to design practical plans that can increase preparedness for earthquakes, Singh argues. “Capable institutions at all levels of government need to carry out the strategies and implement the plans.”
Ake Fagereng, a seismic researcher from the University of Cardiff’s School of Earth and Ocean Sciences, agrees: “If we can improve the local knowledge of seismicity and potential earthquake hazard, and train some local earth scientists to further improve on our models, we are doing well.” Along with researchers from the University of Bristol, Fagereng is completing the PREPARE project in east Africa to help countries in the region with their readiness for potential earthquakes.
“The region has a challenge where magnitude seven or greater earthquakes are possible, but have not occurred in living memory,” says Fagereng. “Similar to Haiti, there is therefore little consideration of earthquake risk.” The project, which is due to run until 2020, is building seismic-hazard-risk maps and design guides with governments and academics in the region.
“What’s needed after an earthquake is for all the critical infrastructure to continue so emergencies are attended,” says Christian Málaga-Chuquitaype, structural engineering lecturer at Imperial College, London. “That means hospitals, fire brigades, key bridges, water supplies, and more.” Such policies can be hard to implement in regions where corruption lurks high up in the political chain and resources are not appropriately distributed.
“Sometimes very short-sighted plans of reconstruction after earthquake events are implemented with only political gains in view, and golden opportunities to build properly adequate sites are missed,” Málaga-Chuquitaype adds. “What is even worse, is that at other times, reconstruction help from the government never arrives and entire communities are left to rebuild their lives as they can.”
Protecting cities and populations from earthquakes is a complex challenge. Experts from the worlds of politics, technology, construction and design are required to work together and think in the long term to keep our built environments standing.